Exposing device and image forming apparatus

ABSTRACT

An exposing device which radiates light on a photoconductor drum which rotates, thereby executing exposure, includes a light-emitting element unit including light-emitting element having an emission area which emits light that is generated, the emission area having a rectangular shape with a long side along a direction perpendicular to a rotational direction of the photoconductor drum, and a short side along the rotational direction, a lens unit which focuses the light, which is emitted from the emission area, on a peripheral surface of the photoconductor drum, thereby executing exposure, and forming on the peripheral surface a beam spot, and a driving circuit which causes the light-emitting element to emit light, thereby making a width of the shape of the beam spot in a direction along the rotational direction close to a width of the beam spot in a direction perpendicular to the rotational direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2007-230575, filed Sep. 5, 2007;and No. 2007-230576, filed Sep. 5, 2007, the entire contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an exposing device whichadopts an electrophotographic method, and an image forming apparatusincluding the exposing device, and more particularly to an exposingdevice using a light-emitting element as an exposure light source, animage forming apparatus including the exposing device, and a drivingcontrol method of the image forming apparatus.

2. Description of the Related Art

Various kinds of electrophotographic image forming apparatuses (printingapparatuses), wherein organic electroluminescence (hereinafter referredto as “organic EL”) elements, for instance, are used as light-emittingelements, have been designed and manufactured as products. This type ofimage forming apparatus is configured such that a light-emitting elementarray, which is composed of a plurality of light-emitting elements, anda photoconductor drum are disposed via a lens, with a predetermineddistance therebetween. Light emitted from each light-emitting element isfocused via the lens, and radiated on the photoconductor, thus formingan electrostatic latent image.

Conventionally, the shape of a part, which emits light from each of thelight-emitting elements of the light-emitting element array, is, ingeneral, substantially squared. Ideally, such design is made that lightemitted from the light-emitting element is focused on the peripheralsurface of the photoconductor drum via the lens, thereby exposing thephotoconductor and forming a substantially circular beam spot on thephotoconductor drum.

A rod lens array, which is composed of a plurality of arrayed rodlenses, is used as the lens that is used in the above-described imageforming apparatus. In the case of the rod lens array, it is verydifficult, for the reason of manufacture, to uniformize the opticalcharacteristics of respective rod lenses, such as refractive indexdistributions and tilt angles of optical axes. Consequently, the shapesof beam spots, which are actually formed on the photoconductor drum viathe respective rod lens of the rod lens array, have some distortionsassociated with the respective rod lenses. As a result, the uniformityin print density corresponding to the respective light-emitting elementsis degraded, leading to non-uniformity in print results.

BRIEF SUMMARY OF THE INVENTION

The present invention has advantages in that in an exposing device whichradiates light on a photoconductor drum and exposes the photoconductordrum, an image forming apparatus including the exposing device, and adriving control method of the image forming apparatus, the influence ofthe distortion of the shape of the beam spot, which is formed on thephotoconductor drum, due to the presence of an interposed rod lensarray, can be suppressed, thereby enhancing the uniformity in printdensity and suppressing occurrence of non-uniformity in a print result.

In order to obtain the above advantages, according to the presentinvention, there is provided an exposing device which radiates light ona photoconductor drum which rotates, thereby executing exposure,comprising: a light-emitting element unit including at least onelight-emitting element having an emission area which emits light that isgenerated, the emission area having a rectangular shape with a long sidealong a direction perpendicular to a rotational direction of thephotoconductor drum, and a short side along the rotational direction,the short side being shorter than the long side; a lens unit whichfocuses the light, which is emitted from the emission area of thelight-emitting element unit, on a peripheral surface of thephotoconductor drum, thereby executing exposure, and forming on theperipheral surface a beam spot having a shape corresponding to the shapeof the emission area; and a driving circuit which controls a timing oflight emission of the light-emitting element of the light-emittingelement unit and causes the light-emitting element to emit light,thereby making a width of the shape of the beam soot in a directionalong the rotational direction close to a width of the beam spot in adirection perpendicular to the rotational direction.

In order to obtain the above advantages, according to the presentinvention, there is provided an image forming apparatus which performsprinting by an electrophotographic method on the basis of image data,comprising: a photoconductor drum which rotates; a light-emittingelement unit including a plurality of light-emitting elements eachhaving an emission area which emits light that is generated, theemission area having a rectangular shape with a long side along adirection perpendicular to a rotational direction of the photoconductordrum, and a short side along the rotational direction, the short sidebeing shorter than the long side; a lens unit which focuses the light,which is emitted from the emission area of each of the light-emittingelements, on a peripheral surface of the photoconductor drum, therebyexecuting exposure, and forming on the peripheral surface a plurality ofbeam spots each having a shape corresponding to the shape of theemission area; and a driving circuit which controls a timing of lightemission of each of the light-emitting elements of the light-emittingelement unit on the basis of the image data, and causes each of thelight-emitting elements to emit light, thereby making a width of theshape of each beam spot in a direction along the rotational directionclose to a width of the beam spot in a direction perpendicular to therotational direction.

In order to obtain the above advantages, according to the presentinvention, there is provided a driving control method of an imageforming apparatus which performs printing by an electrophotographicmethod on the basis of image data, the image forming apparatus includinga light-emitting element unit including a plurality of light-emittingelements each having an emission area which emits light that isgenerated, the emission area having a rectangular shape with a long sidealong a direction perpendicular to a rotational direction of aphotoconductor drum which rotates, and a short side along -he rotationaldirection, the short side being shorter than the long side, the methodcomprising: a step of causing each of the light-emitting elements of thelight-emitting element unit to execute light emission, focusing thelight, which is emitted from the emission area, on a peripheral surfaceof the photoconductor drum via a lens unit, thereby executing exposure,and forming on the peripheral surface a beam spot having a shapecorresponding to the shape of the emission area; and a step ofcontrolling a timing of light emission of each of the light-emittingelements, thereby making a width of the shape of the beam spot in adirection along the rotational direction close to a width of the beamspot in a direction perpendicular to the rotational direction.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 shows a structure example of an image forming apparatus using anexposing device according to a first embodiment of the presentinvention;

FIG. 2 shows a basic structure of an organic EL element according to thefirst embodiment;

FIG. 3 is a perspective view showing the external structure of theexposing device according to the first embodiment;

FIG. 4 is a cross-sectional view showing the structure of the exposingdevice according to the first embodiment, as viewed from the lateralside;

FIG. 5A and FIG. 5B show the structure of a pixel driving circuit of apixel according to the embodiment, and an example of driving waveformsof the pixel driving circuit;

FIG. 6A and FIG. 6B show the structure of a driving circuit for drivingthe pixels of a light-emitting element array according to the firstembodiment, and an example of the driving waveforms of the drivingcircuit;

FIG. 7A and FIG. 7B are a partial schematic view showing the structure,as viewed from above, of those parts of the plural pixels arranged inone block of the light-emitting element array of the first embodiment,which constitute the light-emitting elements, and a cross-sectional viewof this structure;

FIG. 8A to FIG. 8H show the shapes of emission areas and the shapes ofbeams spots radiated on the photoconductor drum, comparing the case inwhich the shape of the emission area is a general conventional one andthe case in which the shape of the emission area is that in the firstembodiment;

FIG. 9A and FIG. 9B are views for explaining the driving principle of apassive matrix driving method, and the structure of a light-emittingelement array according to a second embodiment of the invention;

FIG. 10A and FIG. 10B are views for describing the structure of adriving circuit for driving the light-emitting elements of thelight-emitting element array according to the second embodiment; and

FIG. 11A to FIG. 11H show the shapes of emission areas and the shapes ofbeam spot radiated on the photoconductor drum, comparing the case inwhich the shape of the emission area is a general conventional one andthe case in which the shape of the emission area is that in the secondembodiment.

DETAILED DESCRIPTION OF THE INVENTION

An exposing device, an image forming apparatus (printing apparatus)including the exposing device and a driving control method of the imageforming apparatus, according to the present invention, will now bedescribed in detail, on the basis of embodiments shown in theaccompanying drawings.

First Embodiment

To begin with, a first embodiment of the present invention is described.

FIG. 1 shows a structure example of an image forming apparatus using anexposing device according to the first embodiment of the presentinvention.

The image forming apparatus shown in FIG. 1 includes a photoconductordrum 1, an exposing device 2 in which a case unit 2A and a rod lensarray unit 2B are integrally provided, a charging roller 3, an eraserlight-source photoconductor 4, a cleaning member 5, a developer 6including a developing roller 6 a, a transfer roller 8, a fixing roller9, and a convey belt 11. Reference numeral 7 denotes print paper.

The rod lens array unit 2B is a lens array in which SELFOC™ lenses arearranged in a line or lines, and is a lens unit which focuses incidentlight on the photoconductor drum 1 as an equal-size erect image.

The photoconductor drum 1 is a negative-charge-type OPC (OrganicPhoto-Conductor), and the charging roller 3 is configured as a negativecharger in association with the photoconductor drum 1. As will bedescribed later in detail, the exposing device 2 includes alight-emitting element array which is configured such that a pluralityof light-emitting elements are linearly arranged.

In the image forming apparatus shown in FIG. 1, in, general terms,printing is performed by the following process. To start with, thecharging roller 3 comes in contact with the surface of thephotoconductor drum 1 which rotates. Thereby, the surface of thephotoconductor drum 1, which is put in contact with the charging roller3, is uniformly negatively charged. Then, the exposing device 2 radiateslight on the photoconductor drum 1, and forms an electrostatic latentimage on that area of the photoconductor drum 1, on which light isradiated. Subsequently, by the developer 6, toner is adhered to theelectrostatic latent image. The transfer roller 8 transfers the toner,which adheres to the electrostatic latent image, onto the print paper 7.This printing process will be described below in detail.

To start with, a negative high voltage, which is supplied from acharging power supply (not shown), is applied to the photoconductor drum1 by the charging roller 3. Thereby, the peripheral surface of thephotoconductor drum 1 is uniformly negatively charged, and is set in aninitialized charge state in which the peripheral surface is initializedin terms of potential.

Light corresponding to image data is radiated from the emission area ofthe exposing device 2 onto the photoconductor drum 1 with the peripheralsurface in the initialized charge state, and write (exposure) isexecuted. Thereby, an electrostatic latent image is formed on theperipheral surface of the photoconductor drum 1. The electrostaticlatent image comprises a negative high potential part which is formed bythe initializing charging, and a negative low potential part of, e.g.about −50V, which is formed on an area (beam spot) on which light isradiated by the exposure.

Toner, which is contained in the developer 6 and is charged with a lownegative potential, is conveyed by the rotation of the developing roller6 a to a part at which the developing roller 6 a and the photoconductordrum 1 are mutually opposed. At this time, a developing bias voltage of,e.g. about −250V is applied from a power supply (not shown) to thedeveloping roller 6 a. Accordingly, a potential difference of about 200Vis created between the developing roller 6 a, to which the developingbias voltage of about −250V is applied, and the negative low potentialpart of about −50V of the electrostatic latent image on thephotoconductor drum 1.

By this potential difference from the developing voltage on theelectrostatic latent image, the negatively charge toner is transferredto the negative low potential part of the electrostatic latent image,which has a positive potential relative to the developing roller 6 a,and thus a toner image is formed. The toner image is conveyed by therotation of the photoconductor drum 1 to a transfer part at which thephotoconductor drum 1 and the transfer roller 8 are mutually opposed.

The amount of toner of the formed toner image, that is, the density ofthe developed image, is determined by the amount of attenuation ofpotential on the peripheral surface of the photoconductor drum 1, whichoccurs in accordance with the exposure amount on the photoconductor drum1 by the light-emitting elements of the exposing device 2.

When the toner image is conveyed to the transfer part, as describedabove, the print paper 7 is conveyed to the transfer part by the conveybelt 11. At the transfer part, the toner image is transferred to theprint paper 7 by the transfer roller 8. The print paper 7, to which thetoner image is transferred in this manner, is further conveyed to thedownstream side by the convey belt 11. After the toner image isthermally fixed by the fixing roller 9, the print paper 7 is dischargedto the outside of the image forming apparatus.

After the toner image is transferred onto the print paper 7, theresidual toner is removed from the peripheral surface of thephotoconductor drum 1 by the cleaning member 5. Further, the charge onthe peripheral surface of the photoconductor drum 1 is uniformly erasedto 0V by the eraser light-source photoconductor 4 in preparation for thecharging by the charging roller 3.

In the case unit 2A of the exposing device 2, the light-emitting elementarray is provided. The light-emitting element array is configured suchthat a plurality of light-emitting elements are linearly arranged, forexample, in a line along the axial direction of the photoconductor drum1, which is the main scan direction of the exposure scan on thephotoconductor drum 1 shown in FIG. 1.

Next, the basic structure of an organic EL element, which is applied toeach of the light-emitting elements of the light-emitting element array,is described.

FIG. 2 shows the basic structure of the organic EL element according tothe embodiment.

As shown in FIG. 2, in the organic EL element 20, a pixel electrode(anode) 22, a hole transport layer (HTL) 23, a light-emitting layer (EL)24, and an electron transport layer (ETL) 25 and a counter-electrode(cathode) 26 are formed in the named order or a transparent substrate 21which is formed of, e.g. a glass substrate. Additionally, acounter-substrate (not shown) for sealing these components may beprovided.

It is assumed that the organic EL element shown in FIG. 2 adopts abottom emission structure in which light generated from thelight-emitting layer 24 is emitted from the transparent substrate 21side. Alternatively, the organic EL element may adopt a top emissionstructure in which the light is emitted from the side opposite to thetransparent substrate 21.

The pixel electrode 22 functions as an anode. In the case of the bottomemission structure that is shown, the pixel electrode 22 is formed of atransparent, electrically conductive metal oxide film including atransparent electrode material such as indium thin oxide (ITO) or indiumzinc oxide (IZO).

In this case, the counter-electrode 26 functioning as a cathode isformed of a multilayer reflective structure comprising an electroninjection layer with a low signal function of, e.g. barium, magnesium orlithium, as a lower layer, and a light-reflective metal layer with ahigh signal function of, e.g. aluminum, as an upper layer, or thecounter-electrode 26 is formed of a single layer of a reflective metallayer of, e.g. an aluminum alloy. Thereby, light R from thelight-emitting layer 24 is reflected, as shown in FIG. 2, and, as aresult, the light is emitted to the transparent substrate 21 side. Inthe organic EL element 20, light is emitted to the transparent substrate21 side from the region where the pixel electrode 22 and thecounter-electrode 26 are mutually opposed, and light is emitted from theregion where the pixel electrode 22 and the counter-electrode 26 aremutually opposed. In the present embodiment, as will be described later,such a structure is adopted that an area of light emission is restrictedby providing, for example, a light-blocking film on a part on the lightemission side, and the size and shape of the emission area are set atdesired dimensions.

In the case of adopting a top emission structure instead of the bottomemission structure shown in FIG. 2, the top emission structure isrealized by forming the counter-electrode 26 as an anode and the pixelelectrode 22 as a cathode, and interchanging the above-describedstructural components of the counter-electrode 26 and pixel electrode22.

In this case, the carrier transport layer, which is in contact with thepixel electrode 22, is formed as an electron transport layer, and thecarrier transport layer, which is in contact with the counter-electrode26, is formed as a hole transport layer.

The light-emitting layer 24 includes an organic material whichrecombines holes that are transported from the hole transport layer(HTL) 23 and the electrons that are transported from the electrontransport layer (ETL) 25, thereby generating light.

The organic EL element 20 has been described as adopting the three-layerstructure comprising the hole transport layer 23, light-emitting layer24 and electron transport layer 25. However, the present invention isnot limited to this layer structure. For example, the organic EL element20 may adopt a two-layer structure comprising a hole transport layer andan electron transport layer, a single-layer structure comprising ahole/electron transport layer, a two-layer structure comprising ahole-transporting light-emitting layer and an electron transport layer,or a structure in which some other carrier transport layer is providedbetween these layers. In the present embodiment, the carrier transportlayers, such as the hole transport layer 23, light-emitting layer 24 andelectron transport layer 25, are collectively referred to as “EL layer”.

When a predetermined voltage is applied between the pixel electrode 22and counter-electrode 26, holes are injected in the light-emitting layer24 from the pixel electrode 22, and electrons are injected in thelight-emitting layer 24 from the counter-electrode 26. Thereby, theholes and electrons are recombined in the light-emitting layer 24, andlight is generated.

In the case of the bottom emission structure shown in FIG. 2, the lightR produced by this light generation passes through the pixel electrode22 and transparent substrate 21, and is perfect-diffused and radiated.On the other hand, in the case of the top emission structure, the lightF from the light-emitting layer 24 passes through the counter-electrode26 is perfect-diffused and radiated.

In the above-described electrophotographic image forming apparatus, theexposing device 2 executes write of light on the photoconductor drum 1according to image data. It is difficult to form, by only the case unit2A side of the exposing device 2 without any focusing optical systemstructure, a small-diameter light spot on the photoconductor of thephotoconductor drum 1 which is disposed with a distance of several mmfrom the exposing device 2, and to form a beam spot which resolves a dotof each light-emitting element. Taking this into account, in the presentinvention, the beam spot is realized by combining the rod lens arrayunit 2B with the case unit 2A.

Next, the specific structure of the exposing device 2 is described.

FIG. 3 is a perspective view showing the external structure of theexposing device according to the present embodiment.

FIG. 4 is a cross-sectional view showing the structure of the exposingdevice according to the embodiment, as viewed from the lateral side.

As shown in FIG. 3, the exposing device 2 includes the light-emittingelement array in which a plurality of light-emitting elements arelinearly arranged, for example, in a line, along the main scan directionof recording scan (the axial direction of the photoconductor drum 1).The light-emitting element array includes about 14000 light-emittingelements, assuming that the image forming apparatus can perform printingwith a print density of 1200 dpi over the full width of thephotoconductor drum 1 by setting the vertical direction of print paperof, e.g. A4 size (297 mm (vertical)×210 mm (horizontal)) to accord withthe axial direction of the photoconductor drum 1.

Each of the light-emitting elements of the light-emitting element arrayof the present embodiment is configured to include the organic ELelement 20 shown in FIG. 2.

The organic EL element 20 of each light-emitting element is suppliedwith a control signal which is formed of a pulse voltage according toimage data that is output from a host apparatus (not shown) which isexternally connected to the image forming apparatus. Specifically, thelight emission of each light-emitting element is selectively controlled.The details will be described later.

The organic EL element 20 is electrically connected to theabove-described host apparatus via control cables 31A and 31B, as shownin FIG. 3.

As shown in FIG. 4, the transparent substrate 21 is adhered and fixed toa front case 41, which constitutes a part of the case unit 2A, by anadhesive resin (not shown). A rear case 42, which similarly constitutesa part of the case unit 2A, as shown in FIG. 4, is fitted in the frontcase 41. Specifically, the transparent substrate 21 is sealed and fixedin the case that is constituted by the front case 41 and rear case 42.

A seal lass 27, which constitutes a counter-substrate for sealing theorganic EL element 20, is provided on that surface side of thetransparent substrate 21, where the organic EL element 20 is provided.In addition, a plurality of driver ICs 43 are provided in a manner toavoid the seal glass 27. The driver ICs 43 are electrically connected tothe pixel electrode 22 and counter-electrode 26 shown in FIG. 2.

A sync signal, a clock signal and an image signal are input from acontroller (not shown) to the driver ICs 43. On the basis of thesesignals, the driver ICs 43 control the pixel electrode 22 andcounter-electrode 26.

In the present embodiment, for the reason of the structure of the imageforming apparatus, the exposing device 2 is formed as a single deviceunit. Consequently, at the time of assembly or replacement, someexternal force may act on connection wiring. Thus, cables are separatelystructured between the inside and outside of the case unit 2A, so thatthe cable on the outside of the case unit 2A may have a higher strengthand the workability may be enhanced. For this purpose, as shown in FIG.4, a relay connector 44 is provided on the rear case 42.

An external cable 45 is provided on the outside of the exposing device 2via the relay connector 44, and a relay cable 46 is provided to connectthe relay connector 44 and the driver ICs 43 on the transparentsubstrate 21. Thereby, the external cable 45, relay connector 45 andrelay cable 46 constitute the control cables 31A and 31B shown in FIG.3.

A projection portion 51, which projects toward the photoconductor drum1, is provided on the front case 41. An opening portion is formed in theprojection portion 51, and the rod lens array unit 2B is fitted in thisopening portion so as to face the respective organic EL elements 20. Thegap between the opening portion and the rod lens array unit 2B is sealedwith an adhesive (not shown), and the rod lens array unit 2B is fixed.

Accordingly, even if the front case 41 is not transparent to visiblelight, the light emitted from the organic EL element 20 is made incidenton the rod lens array unit 2B via the sealed space in the projectionportion 51.

The organic EL element 20 shown in FIG. 4 has the bottom emissionstructure in which the transparent substrate 21 faces the rod lens arrayunit 2B. Alternatively, the organic EL element 20 may adopt the topemission structure in which the seal glass 27, which is thecounter-substrate, is disposed to face the rod lens array unit 2B.

Next, the driving method of each light-emitting element of thelight-emitting element array in this embodiment is described.

The light-emitting element array in the present embodiment is configuredto include a plurality of light-emitting elements which are composed oforganic EL elements, and a plurality of pixel driving circuits havingactive elements which are connected to the associated light-emittingelements and drive the light-emitting elements. The respectivelight-emitting elements are driven by an active driving method. Thepixel driving circuit includes, for instance, a thin-film transistor(TFT) as the active element. In this description, one light-emittingelement and one pixel driving circuit, which is connected to thislight-emitting element, are referred to as one pixel, and thelight-emitting element array is configured to include a plurality ofpixels.

FIG. 5A and FIG. 5B show the structure of the pixel driving circuit ofthe pixel according to the embodiment, and an example of drivingwaveforms of the pixel driving circuit.

Specifically, the pixel driving circuit shown in FIG. 5A includes aselect TFT 61, a driving TFT 62 and a storage capacitor 63. The drain ofthe driving TFT 62 is connected to the anode of the organic EL element20.

In the driving waveforms shown in FIG. 5B, a part (1) shows a selectsignal Vselect which is applied to a select line, a part (2) shows avoltage signal Vsource which is applied to a source line, a part (3)shows a signal voltage Vdata which is applied to a data line, and a part(4) shows the waveform of a current flowing in the organic EL element 20via the driving TFT 62.

Specifically, as shown in the part (1) of FIG. 5B, the select signalVselect, which is applied to the select line, is set at a high level ata timing of a select period Tse. In addition, the voltage signalVsource, which is applied to the source line, is set at a high level, asshown in the part (2) of FIG. 5B, and the signal voltage Vdata isapplied to the data line, as shown in the part (3) of FIG. 5B. Then, theselect TFT 61 is turned on and selected by the high-level select signalVselect, and the signal voltage Vdata is written in the storagecapacitor 63. At the same time, the driving TFT 62 is turned on. At thistime, the gate voltage Vgs of the select TFT 61 is determined by theequation,

Vgs=Vdata−Vsource,

in accordance with the value of the signal voltage Vdata that is writtenin the storage capacitor 63, and thus the conductivity of the drivingTFT 62 is determined. During the period in which the voltage signalVsource that is applied to the source line is set at the high level, acurrent corresponding to the conductivity of the driving TET 62 flows inthe organic EL element 20.

Next, a description is given of the circuit structure of the drivingcircuit which drives the respective pixels of the light-emitting elementarray according to the present embodiment, and the driving method of thedriving circuit.

FIG. 6A and FIG. 6B show the structure of the driving circuit fordriving the pixels of the light-emitting element array according to thepresent embodiment, and an example of the driving waveforms of thedriving circuit.

Specifically, FIG. 6A is a block diagram of the driving circuit fordriving the pixels of the light-emitting element array. FIG. 6B showsthe waveforms of respective pulse signals which are applied from a datadriver 71, a select line driver 72 and a source line driver 73, whichare shown in FIG. 6A, to the respective pixels of the light-emittingelement array.

As is shown in FIG. 6A, the light-emitting element array of the exposingdevice 2 is divided into blocks each comprising an n-number ofneighboring pixels (dots), and the light-emitting element array has anm-number of these blocks. Specifically, the total number of pixels(dots), which are arranged on the light-emitting element array, is n×m.The driving circuit is configured to include the data driver 71, selectline driver 72 and source line driver 73.

The select line driver 72 has an m-number of outputs, and is connectedto select lines of the respective blocks. As shown in FIG. 6B, theselect line driver 72 successively applies select signals Vselect1 toVselectm to the select lines of the respective blocks in every 1 linetime which is allocated to 1-line printing with reference to ahorizontal sync signal Hsync (not shown), and successively sets thefirst block, second block, . . . , m-th block in the selected state bythe select TFTs 61. The 1 line time, in this context, is a timeallocated to 1-line exposure on the photoconductor drum 1.

The source line driver 72 has an m-number of outputs, and is connectedto source lines of the respective blocks. As shown in FIG. 6B, thesource line driver 73 successively applies voltage signals Vsource1 toVsourcem to the source lines of the respective blocks in sync with thetiming of the select signals Vselect1 to Vselectm.

The data driver 71 has an n-number of outputs, and is connected inparallel to data lines of an n-number of pixels of each block. As shownin FIG. 6B, the data driver 71 applies signal voltages Vdata1 to Vdatan,which correspond to image data, to each data line during the time periodin which the select signal, Vselect1 to Vselectm, is set at the highlevel, thereby successively writing data corresponding to the image datain the pixels of each block.

Thereby, the light-emitting element of each pixel is on/off controlledonce in every 1 line time 1n sync with these signals.

Next, a description is given of an example of the specific structure andshape of light-emitting elements which are arranged in one block of thelight-emitting element array according to the present embodiment.

FIG. 7A and FIG. 7B are a partial schematic view showing the structure,as viewed from above, of those parts of the plural pixels arranged inone block of the light-emitting element array of the first embodiment,which constitute the light-emitting elements, and a cross-sectional viewof this structure.

In the description below, the parts common to the structural elements ofthe basic structure of the organic EL element 20 shown in FIG. 2 aredenoted by the corresponding reference numerals. FIG. 7A shows thearrangement structure of, e.g. various electrodes of the organic ELelement 20, in the state in which a cathode 87 and an overcoatinsulation film 85, which are described below, are removed. FIG. 7B is across-sectional view taken along line V-V in FIG. 7A.

As shown in FIG. 7B, in the organic EL element 20 a light-blocking film82 is formed on one surface of a transparent substrate 80 (21). Anaperture (opening portion) 81 is formed in the light-blocking film 82,and the light-blocking film 82 is formed of a metallic material which isselected from chromium, a chromium alloy, aluminum, an aluminum alloy,etc. A transparent electrode 84 (22), and a contact portion 84A and ananode 84B which are connected to the transparent electrode 84 (22), areformed on the light-blocking film 82 via a transparent gate insulationfilm 83. An organic EL light-emitting layer 86 (23, 24, 25), whichcomprises a hole transport layer, a light-emitting layer and an electrontransport layer, is provided in an opening portion of an overcoatinsulation film 85 which is provided on the transparent electrode 84 andanode 84B. A cathode 87 (26), which also functions as a reflector and isformed of a metal thin film, is provided on the organic ELlight-emitting layer 86 (23, 24, 25).

When a predetermined voltage is applied between the anode 84R and thecathode 87, holes are injected in the organic EL light-emitting layer 86from the anode 84B, and electrons are injected in the organic ELlight-emitting layer 86 from the cathode 87. Thereby, the holes andelectrons are recombined there, and light is generated. As indicated byan arrow R in FIG. 7B, the light produced by the light generation passesthrough the gate insulation film 83 and aperture 81 and is emitted tothe other surface side of the transparent substrate 80.

The light, which is emitted to the other surface side of the transparentsubstrate 80 via the aperture 81 as described above, is formed as asmall-diameter light spot via the rod lens array unit 2B, as shown inFIG. 1, and is radiated as a beam spot on the photoconductor drum 1.

In the above structure, the light-blocking film 82 may be formed, forexample, by extending the same metal thin film as the gate electrodelayers of the select TFT 61 and driving TFT 62, shown in FIG. 5A, fordriving the organic EL element 20.

The shape of the aperture 81, which is formed in the light-blocking film82, defines the area of that light component emitted to the transparentsubstrate 80 side, which is part of the light that is isotropicallyemitted from the organic EL element 20 of each pixel of thelight-emitting element array. Specifically, the aperture 81 determinesthe size and shape of the area of light emission from the light-emittingelement, and forms the emission area in the present invention.

The aperture 81 (emission area) in the present embodiment is provided ata position opposed to the organic EL light-emitting layer 85. The sizeof the aperture 81 is slightly smaller than the size of the organic ELlight-emitting layer 85, and the shape of the aperture 81 is such arectangular shape that the width in a sub-scan direction, which isperpendicular to the main scan direction, is smaller than the width inthe main scan direction, as will be described later.

In the above structure, the aperture 81 is formed in the light-blockingfilm 82 in order to define the area of light, and this aperture 81 formsthe emission area. Alternatively, instead of providing thelight-blocking film 82, the organic EL light-emitting layer 86 may beformed to have a desired shape or the cathode 87 or anode electrode 84may be formed to have a desired shape or size, thereby defining the areaof light that is emitted from the organic EL element 20 to thetransparent substrate 80 side, and forming the emission area.

Next, a description is given of the relationship between the specificshape of the emission area in the present embodiment and the drivingmethod of the pixel.

FIG. 8A to FIG. 8H show the shapes of emission areas and the shapes ofbeams spots radiated on the photoconductor drum, comparing the case inwhich the shape of the emission area is a general conventional one andthe case in which the shape of the emission area is that in the presentembodiment.

FIG. 8F shows the shape of an emission area PA which is generally usedin the conventional art. As shown in FIG. 8F, the conventional emissionarea PA has a substantially square shape. For example, the width L inthe direction of arrangement of pixels in the light-emitting elementarray, which is the main scan direction corresponding to the horizontaldirection in FIG. 8F, is equal to the width L in the sub-scan directionwhich is perpendicular to the main scan direction and corresponds to thevertical direction in FIG. 8F.

FIG. 8G relates to the case of using a rod lens array having an ideallens structure which causes no distortion. FIG. 8G shows the shape of abeam spot, which is formed, via this rod lens array, on the peripheralsurface of the photoconductor drum 1, that is, the object of lightradiation, by instantaneous short-time light emission of thelight-emitting element, the emission area of which has the shape of theabove-described conventional emission area PA.

FIG. 8H relates to the case of using a rod lens array having an actuallens structure which causes some distortion. FIG. 8H shows the shape ofa beam spot, which is formed, via this rod lens array, on the peripheralsurface of the photoconductor drum 1 by instantaneous short-time lightemission of the light-emitting element, the emission area of which hasthe shape of the above-described conventional emission area PA.

Specifically, in the conventional structure, the light-emitting elementis driven so as to instantaneously emit light only for a short time. Inthe case where the rod lens array is an ideal one, the shape of the beamspot, which is formed at this time on the peripheral surface of thephotoconductor drum 1, is an equal-size erect image and becomes, asshown in FIG. 8G, such a substantially circular shape that both thewidth in the main scan direction (the horizontal direction in FIG. 8G)and the width in the sub-scan direction (the vertical direction in FIG.8G) are equal to the width L of the emission area PA.

However, in the actual rod lens array, many rod lenses are arrayed, andperipheral parts of optical images, which are formed by the respectivelenses, overlap. In addition, the positional relationship between therespective lenses and light-emitting elements is not strictly uniform,and there is non-uniformity in optical characteristics between therespective lenses, such as refractive index distributions and angles ofoptical axes.

Consequently, the shape of the beam spot, which is actually formed onthe peripheral surface of the photoconductor drum 1, has a distortionand is not circular, although the width in the main scan direction andthe width in the sub-scan direction, for example, are approximately L.Specifically, the beam spot has a deformed shape, for example, as shownin FIG. 8h. Moreover, the shapes and areas of beam spots, which areassociated with the emission areas PA of the respective pixels, becomenon-uniform. Since the non-uniform beam spots are successively arranged,this results in non-uniformity in print density.

On the other hand, FIG. 8A shows the shape of an emission area 90A inthe present embodiment.

Specifically, as shown in FIG. 8A, the shape of the emission area 90A inthis embodiment (corresponding to the shape of the aperture 81 in thestructure shown in FIGS. 7A and 7B) Is a rectangular shape having a longside with a width L in the direction of arrangement of pixels in thelight-emitting element array, which is the main scan direction, andhaving a short side with a width M, which is less than the width L, inthe sub-scan direction perpendicular to the main scan direction.

FIG. 8B shows the shape of a beam spot which is formed, via a rod lensarray having an ideal lens structure which causes no distortion, on theperipheral surface of the photoconductor drum 1 by instantaneousshort-time light emission of the light-emitting element, the emissionarea of which has the shape of the emission area 90A according to thepresent embodiment.

FIG. 8D shows the shape of a beam spot which is formed, via a rod lensarray having an actual lens structure which causes some distortion, onthe peripheral surface of the photoconductor drum 1 by instantaneousshort-time light emission of the light-emitting element, the emissionarea of which has the shape of the emission area 90A according to thepresent embodiment.

Specifically, in the case where the rod lens array is an ideal one, theshape of the beam spot, which is formed on the peripheral surface of thephotoconductor drum by instantaneous short-time light emission of thelight-emitting element, becomes an elliptic shape, as shown in FIG. 8B,having the width L in the main scan direction and the width M in thesub-scan direction, the width in the sub-scan direction being less thanthe width in the main scan direction.

On the other hand, the shape of the beam spot, which is formed via theactual rod lens array by instantaneous light emission of thelight-emitting element, has a distortion, as shown in FIG. 8D, due tothe above-described non-uniformity in optical characteristics betweenthe lenses of the rod lens array, although the width in the main scandirection is approximately L and the width in the sub-scan direction isapproximately M.

To cope with this, the present embodiment further includes a structurefor continuously driving the light-emitting element for a predeterminedlight-emission period, instead of causing the light-emitting element toinstantaneously execute light emission for a short time. By thisstructure, the effect of distortion of the beam spot shape due to theactual rod lens array is suppressed.

Specifically, since the photoconductor drum 1 rotates at a fixed speed,if the light-emitting element continuously emits light for apredetermined light emission time, the beam spot, during this lightemission period, moves over the peripheral surface of the photoconductordrum 1 in the sub-scan direction that is perpendicular to the main scandirection. In the present embodiment, the light emission period and theshape of the emission area 90A (the ratio of the short side to the longside) are set so that the width in the sub-scan direction (the verticaldirection in the Figures) of the shape of the beam spot formed on theperipheral surface of the photosensitive drum 1 may become substantiallyequal to the width L in the main scan direction (the horizontaldirection in the Figures) by the continuous light-emission of thelight-emitting element for the predetermined light-emission period.

Thereby, even if there is a distortion in the beam spot shape due to therod lens array, the effect of the distortion can be reduced, theuniformity in print density in each pixel can be improved, and thenon-uniformity in the print result can be decreased. This will beexplained below in greater detail.

FIG. 8C shows, by way of example, the shape of a beam spot which isformed on the peripheral surface of the photoconductor drum 1 when thelight emission by the light-emitting element is continued for apredetermined light-emission period, in the case where the rod lensarray is the ideal one, as shown FIG. 8B, and the elliptic beam spotshape is obtained.

On the other hand, FIG. 8E shows, by way of example, the shape of a beamspot which is formed on the peripheral surface of the photoconductordrum 1 when the light emission by the light-emitting element iscontinued for a predetermined light-emission period, in the case wherethe beam spot shape with a distortion is obtained with the actual rodlens array, as shown FIG. 8D.

The shape of the beam spot, which is formed on the peripheral surface ofthe photoconductor drum 1 in the present embodiment, is determined bythe shape of the beam spot which is formed on the peripheral surface ofthe photoconductor drum by the instantaneous light emission of thelight-emitting element, and the distance by which the beam spot movesover the peripheral surface of the photoconductor drum 1 during thelight-emission period by the rotation of the photoconductor drum 1. Thelight-emission period is set within the 1 line time that is allocated tothe print of one line. The light emission of the light-emitting elementis executed with the cycle of this 1 line period. The light-emissionperiod is so set that the shape of the beam spot, which is formed on theperipheral surface of the photoconductor drum 1 in the 1 line time ofone cycle, may become a shape with an aspect ratio of about 1:1.

In other words, the shape of the emission area 90A and the correspondinglight-emission period are so set that the shape of the beam spot, whichis formed on the peripheral surface of the photoconductor drum 1 in thelight-emission period that is provided in the 1 line time, may becomesuch a shape that the width in the vertical direction that is thesub-scan direction is substantially equal to the width L in thehorizontal direction that is the main scan direction.

In this case, even in the state in which distortion occurs in the beamspot shape by the instantaneous light emission with the rod lens array,as shown in FIG. 8E, the resultant shape of the beam spot formed on theperipheral surface of the photoconductor drum 1 becomes substantiallysimilar to the beam spot shape which is obtained in the case where nodistortion occurs in the beam spot shape by the instantaneous lightemission, as shown in FIG. 8C.

Thereby, even in the case where there is non-uniformity in opticalcharacteristics between the rod lenses that constitute the rod lensarray, it becomes possible to suppress non-uniformity in the beam spotshapes associated with the light-emitting elements of the light-emittingelement array, that is, non-uniformity in print density, and to suppressoccurrence of non-uniformity in the print result.

Next, the relationship between the shape, which is set for the emissionarea 90A, and the light-emission period is explained.

As has been described above, the emission area 90A in the presentembodiment has the rectangular shape having the long side (width L) inthe main scan direction and the short side (width M) in the sub-scandirection that is perpendicular to the main scan direction.

The ratio of the light-emission period to the cycle period correspondingto the 1 line time, that is, the value of the duty ratio of thelight-emission period, and the value of the ratio (M:L) between theshort side and long side of the rectangle of the emission area 90A arecorrelated. In the case where the value of the duty ratio of thelight-emission period is set at a certain value, the ratio between theshort side and long side Of the rectangle of the emission area 90A isdetermined in accordance with the value of the duty ratio. On the otherhand, in the case where the ratio between the short side and long sideof the rectangle of the emission area 90A is set at a certain value, thevalue of the duty ratio of the light-emission period is determined inaccordance with the ratio between the short side and long side of therectangle of the emission area 90A.

Specifically, assume the case in which the cycle period is 1, and thelight-emission period is set at ½ of the cycle period, that is, the dutyratio of 50%. In this case, if the ratio (M:L) of the short side to thelong side of the emission area 90A is 0.5:1, the width in the sub-scandirection of the shape of the beam spot, which is formed in thelight-emission period of ½, becomes 0.5 (corresponding to the width M ofthe short side of the emission area 90A)+0.5 (corresponding to thelight-emission period)=1, with the width of the long side being setat 1. Accordingly, the beam spot having the shape with an aspect ratioof about 1:1 can be obtained.

In addition, assume the case in which the light-emission period is setat ¼ of the cycle period, that is, the duty ratio of 25%. In this case,if the ratio of the short side to the long side of the emission area 90Ais 0.75:1, the width in the sub-scan direction of the shape of the beamspot, which is formed in the light-emission period of ¼, becomes 0.75(corresponding to the width M of the short side of the emission area90A)+0.25 (corresponding to the light-emission period)=1, with the widthof the long side being set at 1. Accordingly, in this case, too, thebeam spot having the shape with an aspect ratio of about 1:1 can beobtained.

Specifically, in the case where the value of the duty ratio of thelight-emission period is set at P, it should suffice if the ratio (M/L)of the short side to the long side of the rectangle of the emission area90A is set at (1−P). On the other hand, in the case where the ratio(M/L) of the short side to the long side of the rectangle of theemission area 90A is set at Q, it should suffice if the duty ratio ofthe light-emission period is set at (1−Q).

As has been described above, in the present embodiment, the shape of theemission area of the light-emitting element of each pixel of thelight-emitting element array is the rectangular shape having the longside in the main scan direction that is the direction of arrangement ofpixels which are arrayed, and having the short side in the sub-scandirection perpendicular to the main scan direction. The ratio betweenthe short side and long side of the rectangular shape is determined onthe basis of the ratio of the light-emission period to the cycle period,i.e. the duty ratio. By the active driving using the active elements inthe pixel driving circuit, control is executed to continuously performexposure for a predetermined light-emission period in the sub-scandirection perpendicular to the main scan direction in accordance withthe rotation of the photoconductor drum 1.

The present invention is not limited to the active driving using theactive elements. Passive driving, for instance, may be adopted ifcontrol can be executed to continuously perform exposure for apredetermined light-emission period.

Thereby, even in the case where the shape of a beam spot which isinstantaneously formed on the peripheral surface of the photoconductordrum 1 has a distortion due to the combination of lenses whichconstitute the rod lens array unit 2B for focusing the emission light ofthe light-emitting element on the peripheral surface of thephotoconductor drum 1, the effect of the distortion can be reduced inthe shape of a resultant beam spot which is formed by continuousexposure on the peripheral surface of the photoconductor drum 1. Hence,the uniformity in print density in each pixel of the light-emittingelement array can be improved, and the non-uniformity in the printresult can be decreased.

In the present embodiment, the case in which the organic EL element 20has the bottom emission structure has been described. Alternatively, theorganic EL element 20 may have a top emission structure. In this case,too, the above-described operation is applicable, and the sameadvantageous effects by this operation can be obtained.

Second Embodiment

Next, a second embodiment of the present invention is described.

To begin with, a driving method of light-emitting elements of alight-emitting element array in this embodiment is described.

The light-emitting elements of the respective pixels in this embodimentare composed by a passive structure which uses no active element. Thelight-emitting elements are driven by a driving method based on apassive matrix driving method.

The circuit structure of the passive matrix driving method for executinglight-emission driving of the organic EL element 20 of eachlight-emitting element, and this driving method are first described.FIG. 9A and FIG. 9B are views for explaining the driving principle ofthe passive matrix driving method, and the structure of thelight-emitting element array according to this embodiment. FIG. 10A andFIG. 10B are views for describing the structure of the driving circuitfor driving the light-emitting elements of the light-emitting elementarray according to this embodiment.

Specifically, FIG. 9A shows the driving principle of the massive matrix.

As shown in FIG. 9A, a massive matrix panel is configured such that aplurality of scanning electrodes and a plurality of signal electrodesare disposed to cross each other, and a plurality of light-emittingelements, which are composed of organic EL elements 20, are arranged ina matrix. In each light-emitting element, an organic EL element 20 iscomposed such that an EL layer is interposed between the scanningelectrode and signal electrode, which function as an anode and acathode, respectively. In this case, if the number of scanningelectrodes is m and the number of signal electrodes is n (m, n=a naturalnumber of 2 or more), a matrix having an (m×n) number of intersectionsis formed, and this number of light-emitting elements can be disposed.

In the driving of this passive matrix panel, a row driver 101 and acolumn driver 102 are provided. Bias voltages are successively appliedfrom the row driver 101 and column driver 102 at a predetermined timingto the scanning electrodes and signal electrodes. Thereby, voltages areapplied to the anodes and cathodes sandwiching organic layers of theorganic EL elements 20 which are formed at the intersections of thescanning electrodes and signal electrodes.

In the present embodiment, the light-emitting element array, which isprovided in the case unit 2A of the exposing device 2 and in which aplurality of light-emitting elements are linearly arranged, is formed byre-arranging the matrix-arrayed light-emitting elements, shown in FIG.9A, in a line or lines. FIG. 9B shows an example of the arrangement ofthe light-emitting elements of the light-emitting element arrayaccording to the present embodiment. In this example, the light-emittingelements are arranged in a single line. In this case, an m-number oflight-emitting elements, which are arranged in the horizontal directionin FIG. 9A, correspond to an m-number of light-emitting elements whichare arranged in the column direction of the (m×n) number ofmatrix-arrayed light-emitting elements shown in FIG. 9A.

FIG. 10A shows the structure of connection between the light-emittingelement array, row driver 101 and column driver 102 in the case wherethe light-emitting elements are structured as shown in FIG. 9B. FIG. 10Bshows a detailed structure of connection between the row driver 101,column driver 102 and each light-emitting element of the light-emittingelement array, which are shown in FIG. 10A.

As shown in FIG. 10A and FIG. 8B, the light-emitting element arraycomprises an n-number of groups (20G1 to 20Gn) each consisting of anm-number of light-emitting elements (organic EL elements 20). Them-number of light-emitting elements, which are included in each group,correspond to an m-number of light-emitting elements which are arrangedin the column direction of the matrix-arrayed light-emitting elementsshown in FIG. 9A. The structure of connection between the row driver101, column driver 102 and each light-emitting element of thelight-emitting element array is substantially equivalent to thestructure of connection, shown in FIG. 9A, between the passive matrixpanel, row driver 101 and column driver 102.

Specifically, signal lines (column 1, column 2, . . . , n), each ofwhich is commonly connected to the m-number of light-emitting elements(organic EL elements 20) of an i-th (i=1, 2, . . . , n) group of thelight-emitting element array, correspond to the plural signal electrodesin FIG. 9A, and are connected to output terminals of the column driver102.

In addition, signal lines (row 1, row 2, . . . , row m), each of whichis commonly connected to a j-th (j=1, 2, . . . , m) light-emittingelement (organic EL element 20) of each group of the light-emittingelement array, correspond to the plural scanning electrodes in FIG. 9A,and are connected to output terminals of the row driver 101.

In the driving by the row driver 101 and column driver 102, the rowdriver 101 successively renders active the row 1 row 1, row 2, . . . ,row m, which correspond to the respective scanning electrodes, duringthe period of the 1 line time which is allocated to 1-line printing withreference to a horizontal sync signal Hsync (not shown), and the columndriver 102 supplies, during this period, data, which is based on imagedata, to the column 1, column 2, . . . , n, which correspond to therespective signal electrodes, thereby controlling the light emissionamount of each light-emitting element (organic EL element 20).

For example, while the row driver 101 renders active the scanningelectrode, row 1, the column driver 102 writes data, which is based onimage data, in the signal electrodes, column 1, column 2, . . . , n,thereby controlling the light emission amounts of the light-emittingelements (organic EL elements 20) which are disposed at theintersections corresponding to the scanning electrode, row 1.

The specific structure of the light-emitting element (organic EL element20) in the present embodiment is, for example, equivalent to thestructure, shown in FIG. 7A, 7B, according to the first embodiment. Thelight-blocking film 82 is similarly provided, and the shape of theaperture 81 that is formed in the light-blocking film 82 determines thesize and shape of the emission area for light emission from thelight-emitting element. The aperture 81 determines the size and shape ofthe area of light emission from the light-emitting element, and formsthe emission area in the present invention.

Instead of providing the light-blocking film 82, the shape of theorganic EL light-emitting layer 86 or the shape of the cathode 87 oranode 84 may be set to a desired shape or size. Thereby, the area oflight, which is emitted from the organic EL element 20 to thetransparent substrate 80 side, may be defined, and the emission area maybe formed.

Next, a description is given of the relationship between the specificshape of the emission area in the present embodiment and the drivingmethod of the pixel.

FIG. 11A to FIG. 11H show the shapes of emission areas and the shapes ofbeams spots radiated on the photoconductor drum, comparing the case inwhich the shape of the emission area is a general conventional one andthe case in which the shape of the emission area is that in the presentembodiment.

FIG. 11F shows the shape of an emission area PA which is generally usedin the conventional art. As shown in FIG. 11F, the conventional emissionarea PA has a substantially square shape. For example, the width L inthe direction of arrangement of pixels in the light-emitting elementarray, which is the main scan direction corresponding to the horizontaldirection in FIG. 11F, is substantially equal to the width L in thesub-scan direction which is perpendicular to the main scan direction andcorresponds to the vertical direction in FIG. 11F.

FIG. 11C relates to the case of using a rod lens array having an ideallens structure which causes no distortion. FIG. 11G shows the shape of abeam spot, which is formed, via this rod lens array, on the peripheralsurface of the photoconductor drum 1, that is, the object of lightradiation, by instantaneous short-time light emission of thelight-emitting element, the emission area of which has the shape of theabove-described conventional emission area PA.

FIG. 11H relates to the case of using a rod lens array having an actuallens structure which causes some distortion. FIG. 11H shows the shape ofa beam spot, which is formed, via this rod lens array, on the peripheralsurface of the photoconductor drum 1 by instantaneous short-time lightemission of the light-emitting element, the emission area of which hasthe shape of the above-described conventional emission area PA.

Specifically, in the conventional structure, the light-emitting elementis driven so as to instantaneously emit light only for a short time. Inthe case where the rod lens array is an ideal one, the shape of the beamspot, which is formed at this time on the peripheral surface of thephotoconductor drum 1, is an equal-size erect image and becomes, asshown in FIG. 11G, such a substantially circular shape that both thewidth in the main scan direction (the horizontal direction in FIG. 11G)and the width in the sub-scan direction (the vertical direction in FIG.11G) are equal to the width L of the emission area PA.

However, in the actual rod lens array, many rod lenses are arrayed, andperipheral parts of optical images, which are formed by the respectivelenses, overlap. In addition, the positional relationship between therespective lenses and light-emitting elements is not strictly uniform,and there is non-uniformity in optical characteristics between therespective lenses, such as refractive index distributions and angles ofoptical axes.

Consequently, the shape of the beam spot, which is actually formed onthe peripheral surface of the photoconductor drum 1, has a distortionand is not circular, although the width in the main scan direction andthe width in the sub-scan direction, for example, are approximately L.Specifically, the beam spot has a deformed shape, for example, as shownin FIG. 11H. Moreover, the shapes and areas of beam spots, which areassociated with the emission areas PA of the respective pixels, becomenon-uniform. Since the non-uniform beam spots are successively arranged,this results in non-uniformity in print density.

On the other hand, FIG. 11A shows the shape of an emission area 90B inthe present embodiment.

Specifically, as shown in FIG. 11A, the shape of the emission area 90Bin this embodiment is a rectangular shape having a long side with awidth L in the direction of arrangement of light-emitting elements inthe light-emitting element array, which is the main scan direction, andhaving a short side with a width M, which is less than the width L, inthe sub-scan direction perpendicular to the main scan direction.

FIG. 11B shows the shape of a beam spot which is formed, via a rod lensarray having an ideal lens structure which causes no distortion, on theperipheral surface of the photoconductor drum 1 by instantaneousshort-time light emission of the light-emitting element, the emissionarea of which has the shape of the emission area 90B according to thepresent embodiment.

FIG. 11D shows the shape of a beam spot which is formed, via a rod lensarray having an actual lens structure which causes some distortion, onthe peripheral surface of the photoconductor drum 1 by instantaneousshort-time light emission of the light-emitting element, the emissionarea of which has the shape of the emission area 90B according to thepresent embodiment.

Specifically, in the case where the rod lens array is an ideal one, theshape of the beam spot, which is formed on the peripheral surface of thephotoconductor drum by instantaneous short-time light emission of thelight-emitting element, becomes an elliptic shape, as shown in FIG. 11B,having the width L in the main scan direction and the width M in thesub-scan direction, the width in the sub-scan direction being less thanthe width in the main scan direction.

On the other hand, the shape of the beam spot, which is formed via theactual rod lens array by instantaneous light emission of thelight-emitting element having the shape of the emission area 90B, has adistortion, as shown in FIG. 11D, due to the above-describednon-uniformity in optical characteristics between the lenses of the rodlens array, although the width in the main scan direction isapproximately L and the width in the sub-scan direction is approximatelyM.

To cope with this, the present embodiment further includes a structurefor driving the light-emitting element by a plural number of times atpredetermined light-emission intervals in the 1 line time allocated to1-line printing, instead of instantaneously driving the light-emittingelement only once. By this structure, the effect of distortion of thebeam spot shape due to the actual rod lens array is suppressed.

Specifically, in the conventional structure, each of the light-emittingelements of the light-emitting element array is configured to emit lightonly once in every 1 cycle time which corresponds to the 1 line time.

By contrast, in the present embodiment, each of the light-emittingelements of the light-emitting element array is configured to emit lightmore than once in every 1 cycle time.

In this case, since the photoconductor drum 1 rotates at a constantspeed, if the light-emitting element emits light by a plural number oftimes, the position of the beam spot, which is formed on the peripheralsurface of the photoconductor drum 1 by each light emission, isdisplaced by a distance corresponding to the movement of the peripheralsurface of the photoconductor drum 1 in each light-emission interval. Asa result, the beam spot, which is formed on the peripheral surface ofthe photoconductor drum 1, has such a shape that a plurality of beamspots, which are formed by a plural number of times of light emission ofthe light-emitting element, overlap with displacements over the distanceof movement of the peripheral surface of the photoconductor drum 1,which corresponds to the light-emission intervals.

In the present embodiment, the number of times of light emission and theshape of the emission area (the ratio of the short side to the longside) are so set that the width in the sub-scan direction (the verticaldirection in the Figures) of the shape of the beam spot formed on theperipheral surface of the photosensitive drum 1 may become substantiallyequal to the width L in the main scan direction (the horizontaldirection in the Figures) by the plural number of times of lightemission of the light-emitting element in the above-described manner.

Thereby, even if there is a distortion in the beam spot shape due to therod lens array, the effect of the distortion can he reduced, theuniformity in print density associated with each light-emitting elementcan be improved, and the non-uniformity in the print result can bedecreased. This will be explained below in greater detail.

FIG. 11C shows, by way of example, the shape of a beam spot which isformed on the peripheral surface of the photoconductor drum 1 when thelight emission by the light-emitting element is executed five times atregular intervals, in the case where the rod lens array is the idealone, as shown FIG. 11B, and the elliptic beam spot shape is obtained byinstantaneous single-time light emission of the light-emitting element.

On the other hand, FIG. 11E shows, by way of example, the shape of abeam spot which is formed on the peripheral surface of thephotoconductor drum 1 when the light emission by the light-emittingelement is executed five times at regular intervals, in the case wherethe beam spot shape with a distortion is obtained with the actual rodlens array by instantaneous single-time light emission of thelight-emitting element, as shown FIG. 11D.

The shape of the beam spot, which is formed on the peripheral surface ofthe photoconductor drum 1 in the present embodiment, is determined bythe shape of the beam spot which is formed on the peripheral surface ofthe photoconductor drum by the instantaneous single-time light emissionof the light-emitting element, the distance by which the peripheralsurface of the photoconductor drum 1 moves during the light-emissioninterval by the rotation of the photoconductor drum 1, and the number oftimes of light emission.

The plural number of times of light emission is executed, for example,at equal intervals. At this time, the light-emission interval is set ata time which is calculated by dividing the 1 line time by the number oftimes of light emission. The shape of the emission area 90B and thenumber of times of light emission are determined so that the shape ofthe beam spot, which is formed on the peripheral surface of thephotoconductor drum 1 by the plural number of times of light emission,may become such a shape that the width in the vertical direction that isthe sub-scan direction is substantially equal to the width in thehorizontal direction that is the main scan direction.

In this case, even in the state in which distortion occurs in the beamspot shape by the instantaneous single-time light emission with the rodlens array, as shown in FIG. 11E, the resultant shape of the beam spotformed on the peripheral surface of the photoconductor drum 1 becomessubstantially similar to the beam spot shape which is obtained in thecase where no distortion occurs in the beam spot shape by theinstantaneous light emission, as shown in FIG. 11C.

Thereby, even in the case where there is non-uniformity in opticalcharacteristics between the rod lenses that constitute the rod lensarray, it becomes possible to suppress non-uniformity in the beam spotshapes associated with the light-emitting elements of the light-emittingelement array, that is, non-uniformity in print density, and to suppressoccurrence of non-uniformity in the print result.

Next, the relationship between the shape, which is set for the emissionarea 90B, and the number of times of light emission is explained.

As has been described above, the emission area 90B in the presentembodiment has the rectangular shape having the long side (width L) inthe main scan direction and the short side (width M) in the sub-scandirection that is perpendicular to the main scan direction.

The value of the number of times of light emission in the cycle timecorresponding to the above-described 1 line time and the value of theratio (M:L) between the short side and long side of the rectangle of theemission area 90B are correlated. In the case where the value of thenumber of times of light emission is set at a certain value, the ratiobetween the short side and long side of the rectangle of the emissionarea 90B is determined in accordance with the value of the number oftimes of light emission. On the other hand, in the case where the ratiobetween the short side and long side of the rectangle of the emissionarea 90B is set at a certain value, the value of the number of times oflight emission is determined in accordance with the ratio between theshort side and long side of the rectangle of the emission area 90B.

Specifically, in the case where the cycle period is 1, if the number oftimes of light emission is 2, the light-emission interval is 0.5. Inthis case, if the ratio (M:L) of the short side to the long side of theemission area 90B is 0.5:1, the width in the sub-scan direction of theshape of the beam spot, which is formed by two-time light emission,becomes 0.5 (corresponding to the width M of the short side of theemission area 90B)+0.5 (corresponding to the light-emissioninterval×1)=1, with the width of the long side being set at 1.Accordingly, the beam spot having the shape with an aspect ratio ofabout 1:1 can be obtained.

In addition, if the number of times of light emission is 4, thelight-emission interval is 0.25. In this case, if the ratio of the shortside to the long side of the emission area 90B is 0.25:1, the width M inthe sub-scan direction of the shape of the beam spot, which is formed byfour-time light emission, becomes 0.25 (corresponding to the width M ofthe short side of the emission area 90B)+0.25×3 (corresponding to thelight-emission interval×3)=1, with the width of the long side being setat 1. Accordingly, in this case, too, the beam spot having the shapewith an aspect ratio of about 1:1 can be obtained.

Specifically, if the value of the number of times of light emission isset at R, it should suffice if the ratio (M/L) of the short side to thelong side of the emission area 90B is set at R. On the other hand, ifthe ratio (M/L) of the short side to the long side of the emission area90B is set at S, it should suffice if 25 the value of the number oftimes of light emission is set at S.

As has been described above, in the present embodiment, the shape of theemission area of the light-emitting element of each pixel of thelight-emitting element array is the rectangular shape having the longside in the main scan direction that is the direction of arrangement ofpixels which are arrayed, and having the short side in the sub-scandirection perpendicular to the main scan direction. The passive matrixdriving is executed in a manner to perform light emission more than onceduring the formation of dots. The ratio between the short side and longside of the emission area is determined in accordance with the number oftimes of light emission during the 1 line time.

Thereby, even in the case where the shape of a beam spot which isinstantaneously formed on the peripheral surface of the photoconductordrum 1 has a distortion due to the combination of lenses whichconstitute the rod lens array unit 2B for focusing the emission light ofthe light-emitting element on the peripheral surface of thephotoconductor drum 1, the effect of the distortion can be reduced inthe shape of a resultant beam spot which is formed on the peripheralsurface of the photoconductor drum 1 by exposure by a plural number oftimes of light emission. Hence, the uniformity in print density in eachpixel of the light-emitting element array can be improved, and thenon-uniformity in the print result can be decreased.

In the present embodiment, the case in which the organic EL element 20principally has the bottom emission structure has been described.Alternatively, the organic EL element 20 may have a top emissionstructure. In this case, too, the above-described operation isapplicable, and the same advantageous effects by this operation can beobtained.

The present invention is not limited to the above-described embodiments.In practice, various modifications may be made without departing fromthe spirit of the invention. The functions, which are executed in theabove-described embodiments, may be properly combined and practiced asmuch as possible. The above-described embodiments include inventions invarious stages, and various inventions can be derived from propercombinations of structural elements disclosed herein. For example, evenif some structural elements in all the structural elements disclosed inthe embodiments are omitted or combined, if advantageous effects can beachieved, the structure without such structural elements can be derivedas an invention.

1. An exposing device which radiates light on a photoconductor drumwhich rotates, thereby executing exposure, comprising: a light-emittingelement unit including at least one light-emitting element having anemission area which emits light that is generated, the emission areahaving a rectangular shape with a long side along a directionperpendicular to a rotational direction of the photoconductor drum, anda short side along the rotational direction, the short side beingshorter than the long side; a lens unit which focuses the light, whichis emitted from the emission area of the light-emitting element unit, ona peripheral surface of the photoconductor drum, thereby executingexposure, and forming on the peripheral surface a beam spot having ashape corresponding to the shape of the emission area; and a drivingcircuit which controls a timing of light emission of the light-emittingelement of the light-emitting element unit and causes the light-emittingelement to emit light, thereby making a width of the shape of the beamspot in a direction along the rotational direction close to a width ofthe beam spot in a direction perpendicular to -he rotational direction.2. The exposing device according to claim 1, wherein the light-emittingelement unit includes a plurality of said light-emitting elements, theplurality of light-emitting elements are linearly arranged, and aplurality of said emission areas are provided in association with theplurality of light-emitting elements and are linearly arranged.
 3. Theexposing device according to claim 2, wherein the lens unit is providedin association with the plurality of emission areas, and is formed of alens array in which a plurality of rod lenses are arranged in an array.4. The exposing device according to claim 1, wherein the driving circuitcauses the light-emitting element to execute light emission, at a timingof a predetermined cycle time, continuously for a light-emission period,which corresponds to the shape of the mission area, within the cycletime.
 5. The exposing device according to claim 4, wherein in a casewhere a ratio of the short side to the long side of the emission area isset at m, the driving circuit sets a ratio of the light-emission periodto the cycle time at (1−m).
 6. The exposing device according to claim 1,wherein the driving circuit causes the light-emitting element torepeatedly execute light emission, at a timing of a predetermined cycletime, by a number of times, which corresponds to the shape of themission area, within the cycle time.
 7. The exposing device according toclaim 6, wherein in a case where a ratio of the long side to the shortside of the emission area is set at n, the driving circuit sets thenumber of times of repetition of light emission, within the cycle time,at n.
 8. The exposing device according to claim 1, wherein thelight-emitting element in the light-emitting element unit is composed ofan organic EL element.
 9. An image forming apparatus which performsprinting by an electrophotographic method on the basis of image data,comprising: a photoconductor drum which rotates; a light-emittingelement unit including a plurality of light-emitting elements eachhaving an emission area which emits light that is generated, theemission area having a rectangular shape with a long side along adirection perpendicular to a rotational direction of the photoconductordrum, and a short side along the rotational direction, the short sidebeing shorter than the long side; a lens unit which focuses the light,which is emitted from the emission area of each of the light-emittingelements, on a peripheral surface of the photoconductor drum, therebyexecuting exposure, and forming on the peripheral surface a plurality ofbeam spots each having a shape corresponding to the shape of theemission area; and a driving circuit which controls a timing of lightemission of each of the light-emitting elements of the light-emittingelement unit on the basis of the image data, and causes each of thelight-emitting elements to emit light, thereby making a width of theshape of each beam spot in a direction along the rotational directionclose to a width of the beam spot in a direction perpendicular to therotational direction.
 10. The image forming apparatus according to claim9, wherein the plurality of light-emitting elements in thelight-emitting element unit are linearly arranged, and the lens unit isprovided in association with the emission areas of the light-emittingelements, and is formed of a lens array in which a plurality of rodlenses are arranged in an array.
 11. The image forming apparatusaccording to claim 9, wherein the driving circuit causes each of thelight-emitting elements to execute light emission, at a timing of apredetermined cycle time, continuously for a light-emission period,which corresponds to the shape of the mission area, within the cycletime, and the cycle time is a time allocated for exposure of one line,which is based on the image data, on the peripheral surface of thephotoconductor drum.
 12. The image forming apparatus according to claim11, wherein in a case where a ratio of the short side to the long sideof the emission area is set at m, the driving circuit sets a ratio ofthe light-emission period to the cycle time at (1-m).
 13. The imageforming apparatus according to claim 9, wherein the driving circuitcauses the light-emitting element to repeatedly execute light emission,at a timing of a predetermined cycle time, by a number of times, whichcorresponds to the shape of the mission area, within the cycle time, andthe cycle time is a time allocated for exposure of one line, which isbased on the image data, on the peripheral surface of the photoconductordrum.
 14. The image forming apparatus according to claim 13, wherein ina case where a ratio of the long side to the short side of the emissionarea is set at n, the driving circuit sets the number of times ofrepetition of light emission, within the cycle time, at n.
 15. The imageforming apparatus according to claim 9, wherein the plurality oflight-emitting elements in the light-emitting element unit are composedof organic EL elements.
 16. A driving control method of an image formingapparatus which performs printing by an electrophotographic method onthe basis of image data, the image forming apparatus including alight-emitting element unit including a plurality of light-emittingelements each having an emission area which emits light that isgenerated, the emission area having a rectangular shape with a long sidealong a direction perpendicular to a rotational direction of aphotoconductor drum which rotates, and a short side along the rotationaldirection, the short side being shorter than the long side, the methodcomprising: a step of causing each of the light-emitting elements of thelight-emitting element unit to execute light emission, focusing thelight, which is emitted from the emission area, on a peripheral surfaceof the photoconductor drum via a lens unit, thereby executing exposure,and forming on the peripheral surface a beam spot having a shapecorresponding to the shape of the emission area; and a step ofcontrolling a timing of light emission of each of the light-emittingelements, thereby making a width of the shape of the beam spot in adirection along the rotational direction close to a width of the beamspot in a direction perpendicular to the rotational direction.
 17. Thedriving control method according to claim 16, wherein the step ofcontrolling the timing of light emission of each of the light-emittingelements includes a step of causing each of the light-emitting elementsto execute light emission, at a timing of a predetermined cycle time,continuously for a light-emission period which corresponds to the shapeof the mission area, and the cycle time is a time allocated for exposureof one line, which is based on the image data, on the peripheral surfaceof the photoconductor drum.
 18. The driving control method according toclaim 17, wherein in a case where a ratio of the short side to the longside of the emission area is set at m, the step of causing each of thelight-emitting elements to execute light emission continuously for thelight-emission period includes a step of setting a ratio of thelight-emission period to the cycle time at (1−m).
 19. The drivingcontrol method according to claim 16, wherein the step of controllingthe timing of light emission of each of the light-emitting elementsincludes a step of causing the light-emitting element to repeatedlyexecute light emission, at a timing of a predetermined cycle time, by anumber of times, which corresponds to the shape of the mission area,within the cycle time, and the cycle time is a time allocated forexposure of one line, which is based on the image data, on theperipheral surface of the photoconductor drum.
 20. The driving controlmethod according to claim 19, wherein in a case where a ratio of thelong side to the short side of the emission area is set at n, the stepof causing each of the light-emitting elements to repeatedly executelight emission includes of a step of setting the number of times ofrepetition of light emission, within the cycle time, at n.